Precipitated silica dispersion

Zinc salt of maleated EPDM rubber in the presence of stearic acid and zinc stearate behaves as a thermoplastic elastomer, which can be reinforced by the incorporation of precipitated silica filler. It is believed that besides the dispersive type of forces operative in the interaction between the backbone chains and the filler particles, the ionic domains in the polymer interact strongly with the polar sites on the filler surface through formation of hydrogen bonded structures. 

The pyrogenic flame hydrolyzed silica Aerosil 200, a commercial product from Degussa, was used as a dispersion in doubly distilled water (1). The precipitated silica was prepared by hydrolysis of orthosilicic acid tetraethylester in ammoniacal solution according to the method of Stober, Fink and Bohn (11). The prepared suspension was purified by repeated centrifugation, separation from solvent and redispersion of the sediment in fresh water. Finally, the water was evaporated and the wet silica dried at 150°C for about half an hour. 

Ionic Medium. Silica dispersions were freshly prepared for each experiment in solutions buffered with 10"3M HC03"/C02. The amount of species dissolved from the amorphous silica surface during the experiment was negligible because of the small rate of dissolution reactions. The ionic medium in which coagulation and adsorption studies were carried out was kept constant I = 1.0 to 2.0 X 10 3M. The conditions in all agglomeration and adsorption experiments were such that no Al(OH)3 precipitated within the period of observation. 

Some materials, among the most porous, show a large volume variation due to mechanical compaction when submitted to mercury porosimetry. High dispersive precipitated silica shows, as low density xerogels and carbon black previously experimented, two successive volume variation mechanisms, compaction and intrusion. The position of the transition point between the two mechanisms allows to compute the buckling constant used to determine the pore size distribution in the compaction part of the experiment. The mercury porosimetry data of a high dispersive precipitated silica sample wrapped in a tight membrane are compared with the data obtained with the same sample without memlM ane. Both experiments interpreted by equations appropriate to the mechanisms lead to the same pore size distribution. 

In the present study, we show that the same pore size distributions obtained on the one hand, by the interpretation of an intrusion curve by the Washburn equation (1) and, on the other hand, by the interpretation of a crushing curve by the buckling equation (2) that we propose, are identical. These results are obtained on samples of industrial high dispersive precipitated silica. 

Mercury porosimetry experiments were performed on a Carlo Erba Porosimeter 2000 allowing measurements in the pressure range 0.01 - 200 MPa. The sample of high dispersive precipitated silica was synthesized and provided by Prayon-Rupel S.A, Belgium. 

In order to identify the volume variation mechanisms on the precipitated silica sample, experiments were performed at various maximum pressure below and near the point of slope change P,.. A monolithic sample of high dispersive precipitated silica was weighted and its specific volume (2.04 cm /g) was determined using mercury pycnometry. It has been submitted to mercury porosimetry until a pressure (40 MPa) just below the characteristic transition... 

Figure 2. Mercury porosimetry curves (Cumulative pore volume versus pressure) obtained on high dispersive precipitated silica samples at maximum experimental pressure 40 MPa (curve a) and 200 MPa (curve b).

Below 45 MPa, the high dispersive precipitated silica sample with or without membrane collapses without mercury intrusion. The buckling mechanism of pores edges can be assumed as in the case of low density xerogels. Consequently, equation (2) can be used to interpret the mercury porosimetry curve in this low pressure domain. The constant A, to be used in equation (2) can be calculated from the P, value using equation (4). With a mercury surface tension 0.485 N/m, a contact angle 0= 130° and P, = 45 MPa, one obtains K = 86.3 nm MPa" . 

Figure 4. Cumulative pore volume distribution versus pore size obtained on high dispersive precipitated silica wrapped in a tight membrane and the same material without membrane.

High dispersive precipitated silica submitted to an increasing pressure in a mercury porosimeter shows successively a collapse mechanism of porous texture followed by a mechanism of mercury intrusion in the part of pore network which has resisted to the collapse. Such a behavior has been previously observed on low density xerogels and on some carbon black. Both mechanisms can be clearly distinguished by a sharp variation of slope of cumulative pore volume curve versus pressure. 

The purification of fine powder precipitated silica is difficult by this method. In some applications of white carbon such as a rubber filler, a certain amount of Na cation remains in order to adjust the acidity of the surface to achieve a better dispersion of particles. The silica products are fairly pure when SiCU and Si(OC2Hs)4 purified by distillation are used. 

Equivalent amounts of FefNO,), and NaOH solutions were added dropwise to a silica dispersion. The dispersion was stirred for 30 min, and aged at 3() C for 1 h. The precipitate was washed, and dried at 105°C for 2 d. 

Another important reinforcement application is in silicone rubber. Historically, fumed silicas have played the major role here, but recently precipitated silicas have been developed that possess the characteristics required for this application (6). Compared to conventional precipitated silicas, a product designed for this end use must have higher purity (to impart acceptable electrical properties, because silicone rubbers are often used as insulating materials) and lower water adsorption (to prevent bubbles from forming during extrusion and to impart resistance against moisture pickup). Good dispersibility is also important. 

Hydrophobic Silicas. Because foaming is a surface phenomenon, any antifoam used must concentrate at the surface (or gas—liquid interface). Hydrophobic silicas, which are silicas that have been treated with a compound that causes them to float on the top of water, have been used to fulfill this function for almost 30 years. U.S. Patent 3 408 306 (5) discloses the use of a hydrophobic silica dispersed in a hydrocarbon oil. Hydrophobic silica for this composition, which is still in use today, is made either by continuous ( dry roast ) or batch process. In either process, precipitated silicas rather than silica gels or fumed silicas are typically used to make antifoams. During a continuous process, silicone oils, usually poly(dimethylsiloxane), are sprayed onto a bed of hydrophilic silica. The bed is heated to temperatures ranging up to 300 °C, and reaction times are up to 20 h. At these temperatures and reaction times, bond formation between the silica particle and silicone oil may occur in addition to simple coating of the particle. 

Figure 4 shows a hydrophilic and the corresponding hydrophobic precipitated silica mixed with dimethylpolysiloxane. The mixture with the hydrophilic silica gives an almost solid compoimd. In contrast, the well hydrophobized silica leads to a liquid silica-in-oil dispersion. 

The determination of the sieving residue (ISO 787/18) provides an indication of the difficult to disperse amount of the fractions in a precipitated silica. 

In The Chemistry of Silica [1], Her used the term silica powders as a broad category encompassing silica gels, precipitated silicas, and fumed or pyrogenic silicas. These are all forms of synthetic amorphous silicon dioxide, a broad category that also includes another form — silica sols or coUoidal silica — that is not a powder, but rather a dispersion of discrete silica particles in a liquid medium. 

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